All clinical nuclear medicine imaging, both PET and single photon, is done exclusively with crystal detector systems. These detectors impose a host of limitations in both cost and technical performance. In 140 keV imaging, the NaJ/PMT camera, the workhorse of nuclear medicine, is extremely bulky, costly, and limited in both count rate and spatial resolution. In 511 keV PET imaging, exotic high Z crystals must be employed leading to very high cost and very limited solid angle. Under HL59805, PTI has developed a practical small tubular high pressure xenon detector which can operate in sealed mode for years at a density of 0.55 g/cm3. This medium has the potential to produce a 10-fold energy resolution improvement over Nal and LSO and a time resolution comparable to LSO. We propose, as an extension of the PTI small tubular detector, a larger cylindrical pulse ionization detector (20-50 mm in diameter) equipped with a segmented cathode strip structure and a tight transmitting end window. Extensive pilot analytical studies indicate that, through use of the strip cathode electrode signal distribution, a general purpose detector element can be achieved capable of both 140 keV and 511 keV imaging and having excellent 3-D spatial resolution on the order of 1 mm. Pilot experimental studies indicate that, through use of light signals produced by both the primary interaction process and stimulated emission near the electron collection point at the anode, energy resolution approaching amplifier noise limits is possible. Thus, for an amplifier noise of 50 e- rms, energy resolution at 140 keV can be under 2 percent FWHM and significantly better at 511 keV. The density of xenon employed is about 6-fold less than Nal but still affords efficient detection of 140 keV in a suitably thin detector. For 511 keV detection, the multiple interaction vertices which occur in xenon are adequately spread out among distinct tubes in an absorbing array and primary scintillation light provides coincidence time resolution of I ns. Thus, the proposed detector element configured in appropriate arrays can offer greatly improved performance in both of the major nuclear medicine imaging arenas. In Phase I, feasibility and functional spatial and energy resolution limits will be established through construction and testing of prototypes. In Phase II, a fully functional detector element will be developed and operated in small arrays to evaluate practical clinical imaging applications. PROPOSED COMMERCIAL APPLICATION: The current application proposes development of a novel high pressure xenon radiation detector element that will offer substantial improvements in spatial and energy resolutions for energies including 140 keV and 511 keV. Thus, this technology could provide a high performance, durable, and relatively low cost radiation detection medium for use in many nuclear imaging technologies, including PET, collimated single photon imaging, and Compton imaging. Because this technology could replace the basic detection element in a broad range of applications, it has a very large potential commercial market.